1. Field of the Invention
This invention relates to a lens control device advantageously adapted for an image pickup apparatus such as a video camera or the like.
2. Description of the Related Art
The advancement and popularization of image pickup devices such as video cameras and the like have been widespread during recent years. The popularization of them has prompted many improvements and diversification of functions. Among these improvements, automatic focusing has become indispensable as a function, obviating the necessity of a focusing operation, a be the most troublesome operation.
Methods of varied kinds have been employed for automatic focusing devices. One of these methods called a xe2x80x9chill climbingxe2x80x9d automatic focusing method is becoming most popular. In the hill climbing automatic focusing method, a signal such as a high-frequency component which indicates sharpness and varies with the state of focus is extracted from a video signal outputted from image pickup means which converts a picked up image into the video signal. With the state of focus thus detected from the sharpness-indicating signal, a focus lens is driven to bring the level of the sharpness-indicating signal to a maximum level. In accordance with this method, an infocus state can be determined from information obtained within a pickup image plane irrespective of the distance to an object of shooting. Other advantages of this method include a point that, unlike a so-called active automatic focusing method of illuminating the object with infrared rays or the like and detecting the rays thus reflected, this method dispenses with mechanisms such as an infrared ray emitting part and a light receiving part.
However, an automatic focusing (AF) method such as the above-stated method of using a video signal outputted from image pickup means and driving a focus lens in a direction in which the level of the high frequency component of the video signal comes to reach a peak level presents a problem in the following point. It is possible to find the direction in which the focus lens is to be moved from its current position for an in-focus position, i.e., in which the level of the high-frequency component reaches its peak level, when the focus lens is moving. If the focus lens is not moving, however, it is impossible to find the direction of the in-focus position on the basis of the level value of the high-frequency component alone.
In view of this problem, a direction determining method is employed. According to this method, the focus lens is forcibly vibrated at a predetermined amplitude in the direction of an optical axis, either constantly or only at the time of a start, and the direction in which the focus lens is to be driven is decided according to changes taking place in the level of the high-frequency component of the video signal. The vibration of the focus lens used in determining the direction is called wobbling.
The focus lens driving direction can be correctly decided by carrying out wobbling in this manner. However, there is another problem.
Changes in the high-frequency component, i.e., the sharpness of an image obtained through wobbling are affected by the influence of the depth of field. It is known that the amount of changes taking place in sharpness resulting from the same focus lens driving amount comes to vary when the depth of field of an optical system varies. For the same amplitude of wobbling, the sharpness varies to a greater extent accordingly as the depth of field is shallower and to a lesser extent accordingly as the depth of field is deeper. It has been proposed as a solution of this problem to increase the amplitude of wobbling when the depth of field is deep and to decrease the former when the latter is shallow, in such a way as to have the direction decided always at the same sensitivity.
However, this solution is still insufficient for some shooting conditions. For automatic exposure control, an aperture value in the video camera is automatically controlled to have the video signal at a constant level. When the shooting conditions yield a low illuminance, the position of an iris is automatically shifted to a full open position. Then, if the illuminance is found to be still insufficient, an automatic gain control (AGC) circuit comes to increase its rate of gain. As a result, the level of the video signal level is amplified to give an apposite exposure (an apposite signal level).
With the device arranged in the manner described above, the amplitude of wobbling cannot be changed and is left unvarying after the iris is fully opened. Therefore, the gain is increased by the AGC circuit while the amplitude of wobbling is left unvarying. However, as well known, the AGC circuit is a source of a noise. The noise is inevitably increased when the gain of the AGC circuit is increased. Meanwhile, the amplitude of wobbling is normally set to be sufficiently small to be indiscernible by visual sensations. Therefore, the changes introduced into sharpness by wobbling tend to be prevented by the noise from being accurately detected. Such a condition presents a serious problem as it tends to result in an erroneous decision.
FIG. 8 shows characteristic curves representing changes taking place in the gain of the AGC circuit and changes in an S/N ratio caused by the changes of the gain of the AGC circuit as in relation to the luminance of the object. Referring to FIG. 8, as shown by the curve 201, the gain is at xe2x80x9c1xe2x80x9d and is unvarying when the object is in a bright state as indicated on the right side of a point 202. However, when the object is in a low luminance state, on the left side of the point 201 as viewed on the drawing, where exposure control cannot be adequately carried out with the iris alone, the gain of the AGC circuit increases and, on the other hand, the S/N ratio is degraded. With the S/N ratio thus decreased, a focus voltage which is a sharpness signal (or the high frequency component of the video signal) changes in a manner as represented by a curve 401 in FIG. 9. Referring to FIG. 9, the focus voltage reaches a maximum value when the focus lens is in an in-focus position. The level of the focus voltage then lowers accordingly as the focus lens moves further away from the in-focus position. The noise comes to be more conspicuously superimposed on the focus voltage accordingly as the focus voltage becomes lower. The adverse effect of the noise then becomes so conspicuous that it becomes impossible to correctly decide the focus lens moving direction. The probability of making an erroneous decision increases under such a condition.
This invention is directed to the solution of the problems mentioned in the foregoing.
It is a first object of this invention to provide a lens control device which is arranged to be capable of correctly deciding the focus lens moving direction irrespective of shooting conditions.
It is a second object of this invention to provide a lens control device which is capable of accurately carrying out an automatic focusing action even when the object of shooting is at a low luminance.
It is a third object of this invention to provide a lens control device which is capable of always accurately carrying out an automatic focusing action without being affected by an adverse influence of an automatic gain control (AGC) circuit.
To attain these objects, a lens control device arranged as a preferred embodiment of this invention comprises focus control means arranged to detect an amount of change in the level of a video signal by driving a lens at a predetermined amplitude and to move the lens toward an in-focus point on the basis of the amount of change detected, level adjusting means for adjusting the level of the video signal, detecting means for detecting an amount of adjustment of the level of the video signal by the level adjusting means, and amplitude control means for varying the predetermined amplitude of the driving of the lens according to the amount of adjustment of the level detected by the detecting means.
With the lens control device arranged in the above-stated manner, the level adjusting amount of the level adjusting means arranged to adjust the level of the video signal is detected by the detecting means. The amplitude of the lens driving action is changed by the amplitude control means according to the amount of level adjustment. The lens is driven at the changed amplitude. Then, the amount of change taking place in the level of the video signal is detected. A direction in which the lens is to be moved is decided on the basis of the detected amount of change. The lens is moved in this direction. By this, focus control can be accomplished by the lens control device without being affected by noises.
It is a fourth object of this invention to provide a lens control device which is capable of performing automatic focusing control in an optimum manner by varying a focus lens driving characteristic according to the gain of automatic gain control (AGC).
It is a fifth object of this invention to provide a lens control device which is capable of performing automatic focusing control with an improved accuracy in the event of low illuminance.
These and other objects and features of this invention will become apparent from the following detailed description of embodiments thereof taken in connection with the accompanying drawings.